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题目材料：
In 1644 Descartes described Earth as consisting of a central nucleus of hot primordial fluid surrounded by a solid opaque layer and then succeeding layers of rock, metal, water, and air. Geophysicists still subscribe to the notion of a layered Earth. In the current view, however, Earth possesses a solid inner core and a molten outer core, both consisting of iron-rich alloys at enormous temperatures and pressures, followed, at about 2,900 kilometers below the surface, by a mantle of solid, less dense magnesium-iron silicates; the boundary between the upper and lower mantle lies 670 kilometers below the surface. At 30-50 kilometers below the continental surface (less than 10 kilometers below the seafloor), the Mohorovicic discontinuity marks an additional boundary--that between the mantle and the less dense crust above it.
以上解析由 考满分老师提供。
